The present invention relates to the field of magnetic storage devices. More particularly, the present invention relates to a synthetic ferrimagnet reference layer for a magnetic storage device.
Magnetic Random Access Memory (MRAM) is a non-volatile memory that has lower power consumption than short-term memory such as DRAM, SRAM and Flash memory. MRAM can perform read and write operations much faster (by orders of magnitude) than conventional long-term storage devices such as hard drives. In addition, MRAM is more compact and consumes less power than hard drives.
A typical MRAM device includes an array of memory cells, word lines extending along rows of the memory cells, and bit lines extending along columns of the memory cells. Each memory cell is located at a cross point of a word line and a bit line.
The memory cells may be based on tunneling magneto-resistive (TMR) devices such as spin dependent tunneling junctions (SDT). A typical SDT junction includes a pinned layer, a sense layer and an insulating tunnel barrier sandwiched between the pinned and sense layers. The pinned layer has a magnetization orientation that is fixed so as not to rotate in the presence of an applied magnetic field in a range of interest. The sense layer has a magnetization that can be oriented in either of two directions: the same direction as the pinned layer magnetization or the opposite direction of the pinned layer magnetization. If the magnetizations of the pinned and sense layers are in the same direction, the orientation of the SDT junction is said to be xe2x80x9cparallel.xe2x80x9d If the magnetizations of the pinned and sense layers are in opposite directions, the orientation of the SDT junction is said to be xe2x80x9canti-parallel.xe2x80x9d These two stable orientations, parallel and anti-parallel, may correspond to logic values of xe2x80x980xe2x80x99 and xe2x80x981xe2x80x99.
The magnetization orientation of the pinned layer may be fixed by an underlying antiferromagnetic (AF) pinning layer. The AF pinning layer provides a large exchange field, which holds the magnetization of the pinned layer in one direction. Underlying the AF layer are usually first and second seed layers. The first seed layer allows the second seed layer to be grown with a crystal structure orientation. The second seed layer establishes a crystal structure orientation for the AF pinning layer.
The pinned layer in some conventional magneto-resistive memory devices may have a net magnetic moment, which leads to undesirable effects. One such effect is that of a demagnetizing field. For example, the magnetic layer of the pinned layer reaches and interacts with the sense layer. As the sense layer stores information by the orientation of its magnetization, clearly its magnetic orientation must be preserved. Thus, the interaction of the magnetic field from the pinned layer may lead to loss of data if this magnetic field becomes too strong. A second problem is that the presence of the magnetic field from the pinned layer requires that an asymmetric magnetic field be used to switch the state of the data layer, which adds to the complexity of the writing process. A still further problem is that the tolerance for stray magnetic fields during writing is lowered.
As it is desirable to fabricate high capacity memories, it is desirable to fabricate an array of such memory cells as dense as possible. Unfortunately, the cumulative demagnetizing effects of all of the reference layers may constrain how densely the memory cells may be packed.
Another disadvantage of pinned structures is that the materials needed to achieve pinning (e.g., the AF pinning layer and the seed layer) are both complicated and expensive to fabricate.
Therefore, a need exists for an information storage device using magneto-resistive memory cells. A further need exists for such a device that minimizes a demagnetizing field that may be present in conventional magnetic storage devices. A still further need exists for a device that may be fabricated more economically and with fewer and simpler materials than conventional magnetic storage devices.
Accordingly, embodiments of the present invention provide a synthetic ferrimagnet reference layer for a magnetic storage device. Embodiments of the present invention provide for a device that minimizes a demagnetizing field that may be present in conventional magnetic storage devices. A synthetic ferrimagnet reference layer for a magnetic storage device is disclosed. The reference layer has first and second layers of magnetic material operable to be magnetized in first and second magnetic orientations. A spacer layer between the layers of magnetic material is of suitable dimensions to magnetically couple the magnetic layers in opposite directions. The layers of magnetic material have substantially the same coercivities.